In the conventional power conversion system, AC-DC converters are used to convert the AC input voltage VAC supplied by the grid power into a desired DC voltage VDC, and after the voltage converter modulated the voltage VDC, producing the final DC output voltage VOUT with small ripple. The conventional technique employed by the AC-DC conversion systems of prior art faces a problem is that the peak value or the effective value, which is also called the root mean square (RMS) value, of the AC input voltage VAC is not constant at all times. When the AC input voltage VAC from the mains electricity is in under voltage or over voltage condition, such as shown in FIG. 1 when the DC voltage V in the period T0 falls below the minimum power supply voltage, for example, usually resulting in flickering or dimming of the display, which may cause damage to the AC-DC converter. Thus real-time monitoring and determination of the trend of the AC input voltage VAC is essential.
In U.S. Patent Application US20090141523, two series resistors are used to form a voltage divider and the detection voltage which reflects the changes in the input voltage VAC is generated at the common node between the two resistors. The two resistors are connected in series between the DC output VM of the AC input voltage VAC power supply and the ground, which is well known in the art, and are turned on thus generating a current flowing through the two resistors, as a result, the resistors consume power, even though the two resistors merely serve as auxiliary detection components. In view of this, the requirement of a device that can effectively detect changes in input voltage VAC, accurately reflects the trends of voltage VAC, but at the same time able to avoid unnecessary excessive power consumption is a big challenge.
Furthermore, in order to filter out high frequency noise originated from the AC voltage, existing technology often employs connecting a high frequency filter capacitor CX to the input terminal of the power conversion device that supplies AC voltage. This implementation introduces negative impact as well. When the AC power is removed, the high frequency filter capacitor CX will retain a residue DC voltage that equals to the AC voltage at the instance of removal. Unfortunately, if the AC power is removed at the peak of the AC voltage, the residue DC voltage equals the maximum AC voltage, which can easily cause a risk of an electric shock. The conventional method for discharging the residue voltage is using a bleeding resistor RB connecting in parallel to the high-frequency filter capacitor CX. However the use of the bleeding resistor RB would result in power loss, which can be calculated by:P=(VIN_RMS)2/RB,where VIN_RMS is the effective value or the root mean square value of the AC power.
The power loss resulted from bleeding resistor RB is a serious matter for AC-DC converter, especially under no-load or standby condition. Hence it is a challenge to determine the instance to remove the AC power and to discharge the residue voltage without increasing power loss.
It is within this context that embodiments of the present invention arise.